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On Earth, our atmosphere contains 78.09% nitrogen, 20.95% oxygen, on average around 1% water vapor at sea level, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. Looking through this list not everything seems necessary for a habitable atmosphere.

  • Nitrogen is needed by plants, but not in its gaseous form so supplying this rare resource which needs to be mined on Venus, Earth, Titan, from the depths of the giant planets or the sun vis water bound fertilizers seems more appropriate. Nitrogen can go

  • Oxygen is obviously required for respiration on al level between 0.16 and 0.5 atm. In order to keep respiration easy and activity comfortable, I will keep it slightly below the Earth standard. Oxygen is required

  • Water vapor is unavoidable in an inhabited section due to evaporation. There will be water vapor

  • Argon is a product of radioactive decay. Argon be-gone

  • Carbon dioxide is produced by animals and needed for plants. The Carbon dioxide needs to be taken care of either by scrubbers or plants.There will be Carbon Dioxide

Since any spacecraft is subject to the rocket equation reducing mass is always a good thing. While using a bare minimum atmosphere will only save minuscule amounts of mass any gram counts on a space vessel. Furthermore, lower pressure will reduce the amount of gas escaping into space.

Thus my first idea is a pure 0.2 atm oxygen atmosphere for most spacecraft. I this plausible? NASA used a pure Oxygen atmosphere for the Apollo missions and it worked fine. Sure there was the Apollo fire but that happened in a 1.14 atm pure oxygen atmosphere so this is a different environment. NASA used 0.35 atm on the actual flights, but I don´t understand why they used so much pressure. Shouldn´t the chance of getting the required oxygen out of an Earth-like atmosphere with 0.2 atm be the same as getting the oxygen in a pure 0.2 atm oxygen atmosphere? Or should I increase my pressure to the NASA approved value of 0.35 atm?

On space stations I would like to use an Oxygen/Helium atmosphere. Helium instead of Nitrogen because it is dirt cheap since it is a waste product of the extensive Metallic Hydrogen, Helium3, and Deuterium mining operations. Nitrogen is expansive because it is harder to get and used for agriculture. The other advantages of Helium are that is light, seems to have no adverse effects of the human body and transmits heat very well (six times better than regular air) allowing me to achieve thermal properties similar to regular air with a lot less Helium. Of cause, there is the "Helium Voice" due to the much higher speed of sound in Helium, but that´s something one can get used to over time. The station air mix would be 0.2 atm Oxygen and 0.15 atm Helium.

Of cause, not all station and vessels in my setting these mixtures but they are the most economical and thus the most common. Nitrogen/Oxygen does exist yet is only used on planets and near planet stations.

Are these atmospheres plausible? Did I mess up somewhere? Can this be improved?

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  • $\begingroup$ Are you sure that the Apollo fire happened in a 1.01 atm pure oxygen atmosphere? That would be 5 times the "normal" oxygen pressure, and it's already in the pulmonary toxicity region. $\endgroup$
    – L.Dutch
    May 21 '19 at 5:27
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    $\begingroup$ @L.Dutch After the hatches were sealed, the air in the cabin was replaced with pure oxygen at 16.7 psi (115 kPa), 2 psi (14 kPa) higher than atmospheric pressure. There was definitely overpressure in order to test the behaviour of a pure oxygen atmosphere at a higer pressure that the environment of the capsule and it was higher that 1 atm. Every website I look up gives me a slightly different value... My guess would be that pressure and exposure time matter, in facht one if the astronauts even mentioned a sour smell in the capsule. $\endgroup$ May 21 '19 at 7:07
  • $\begingroup$ "NASA used a pure Oxygen atmosphere for the Apollo missions and it worked fine".. the (ex) crew of Apollo 1 wish to disagree with you. $\endgroup$
    – user79911
    Nov 19 '20 at 20:08
  • $\begingroup$ @L.Dutch-ReinstateMonica 1 bar of pure oxygen only begins to show effects after many hours. They were using pure O2 on ground so they could just vent the overpressure to get to the planned cabin atmosphere in space. $\endgroup$
    – Karl
    Nov 19 '20 at 21:09
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First of all, your methodology is sound. Atmospheres are about partial pressure and setting up an in-ship atmosphere of around 0.2 to 0.3 ATM of pure oxygen is reasonable and actually practical; It means that your ship doesn't have as much differential pressure to deal with against the vacuum outside, meaning it an be lighter as the walls can be a little thinner. Of course, you still have to deal with debris strike or combat situations potentially, but if you're in an environment where that's likely you'd actually have the ship go vacuo instead and have everyone in space suits; That way, a hull breach doesn't kill everyone and send the ship off in strange vectors as a result of the gas venting.

Helium is an inert gas, so would work to 'thin' your oxygen on stations. Yes, everyone would have 'Alvin' voices but there are ways to remodulate spoken words for people in such an environment. But, what I'm not sure about (and have never researched) is if you even need the atmosphere to be that thick on a station. The Nepalese and Tibetan people seem to live in much thinner atmospheric pressure than we do for their entire lives with little difficulty, so I'm not sure if there are any long term effects to living in 0.3 ATMs of pure O2 for your entire life but it might be worth looking up. If not, then running your stations on the same mix as your ships would solve a lot of problems in terms of docking and the like when it comes to equalising pressure and extracting the helium for ship supplies.

Also, I know helium is used by the medical fraternity in MRI scanners, so I can imagine in an environment where there could be a lot of medical issues coming through the doors, doctors would love to get their hands on the kind of helium you can generate as a waste product and therefore would rather you bottle it than release it into the air as a thinner.

In short, your thinking is sound but the only improvement that I can see is that I'd be researching the idea of keeping your stations on a low pressure environment as well for simplicity and ease of docking, if it doesn't cause long term complications.

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  • $\begingroup$ Whoever can afford an MRI scanner also has the money for the helium. ;) And all modern clinical scanners are crygen-free anway (meaning they only need a He refill after years). $\endgroup$
    – Karl
    Nov 19 '20 at 21:22
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You absolutely need some form of buffer gas mixed with the oxygen. In your case you propose Helium.

This will work, mostly.

However: Helium penetrates everything. It enters your body's cells. It enters electonics. It enters pure chemical samples. It dissolves in even steel. And then un-dissolves itself on the other side, escaping your pressure vessel.

Usually the helium causes no problem when it ends up in the wrong place, it is a very inert chemical. But sometimes, it is an issue. Some electronics, some biological systems, etc. do not appreciate high concentrations of Helium.

There's a lot of discussion of this on the related question at What would be the consequences of an Earthlike Planet retaining helium in its atmosphere

Plus, of course, everyone will SOUND LIKE CARTOON CHIPMUNKS!

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  • $\begingroup$ Yea, helium contents that easily occur in a room with a cryomagnet disabled at least one iPhone version. The phones came back to life after a few days for most people. ;) Which biological system cares about He? $\endgroup$
    – Karl
    Nov 19 '20 at 21:13
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You don't want pure oxygen atmospheres. For the ability to breathe, partial oxygen and carbon dioxide pressure is what matters. For fire, the worst danger in any enclosed system, the stoichiometric ratio of gasses is what matters, so a filler gas will prevent fatal accidents.

0.2 atm of oxygen may burn, while adding 0.8 atm of filler may inhibit a fire. The filler is not used up, so nitrogen can be used, Argon can also be considered. The filler gas can be considered part of the dry mass of the ship.

With helium as a filler gas could work, however you should consider that it may separate from oxygen in centrifugal gravity in any space where you don't have enough flow. Helium is also pretty good at diffusing through otherwise gas tight walls, and the squeaky voices will need to get used to.

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    $\begingroup$ Helium’s escape artistry is probably the most concerning point. Atmospheric losses over time would become considerable. $\endgroup$
    – Joe Bloggs
    May 21 '19 at 10:46
  • $\begingroup$ I microgravity, fires don't spread like they do on earth. And NO, a centrifuge that noticeably separates He from O2 is not a place for mammals. Not even very small mice. $\endgroup$
    – Karl
    Nov 19 '20 at 21:19

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